Embryonal Rhabdomyosarcoma: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 11, 2026

Embryonal rhabdomyosarcoma is a malignant (cancerous) tumor that arises from immature skeletal muscle cells. It is classified as a sarcoma and is the most common type of rhabdomyosarcoma, accounting for about 60–70% of all cases. This cancer predominantly affects children, typically those under the age of 10, and most commonly arises in the head and neck region or the genitourinary tract. It can also develop in the extremities, trunk, and other sites.

This article will help you understand the findings in your pathology report — what each term means and why it matters for your care. If you have been diagnosed with a different subtype of rhabdomyosarcoma, our article on alveolar rhabdomyosarcoma may also be helpful.

What causes embryonal rhabdomyosarcoma?

The exact cause is not fully understood. Embryonal rhabdomyosarcoma arises from genetic changes that disrupt the normal growth and development of immature skeletal muscle cells, but these changes are not inherited and are not passed down through families. Certain inherited genetic syndromes increase the risk of developing this cancer, including Li-Fraumeni syndrome (caused by mutations in the TP53 gene), Beckwith-Wiedemann syndrome, and Costello syndrome. Exposure to radiation may also increase risk in some cases. For most children with embryonal rhabdomyosarcoma, no specific cause or predisposing condition is identified.

It is important to note that the PAX3–FOXO1 fusion gene — which is the defining molecular change in alveolar rhabdomyosarcoma — is not present in embryonal rhabdomyosarcoma. The absence of this fusion is one of the key molecular features that distinguishes the two subtypes.

What are the symptoms?

The symptoms of embryonal rhabdomyosarcoma depend on the tumor’s location. Because the tumor can grow quickly, symptoms may worsen over weeks to months. General symptoms such as fatigue, weight loss, or fever can occur if the cancer has spread.

• Head and neck — A visible or palpable mass, nasal congestion, bleeding, swallowing difficulties, or bulging of the eye (proptosis).
• Genitourinary tract — Difficulty urinating, blood in the urine, a palpable mass in the abdomen, or in girls, a mass that protrudes from the vagina.
• Extremities and trunk — A lump or swelling, pain, and limited range of motion.
• Near the spine or in the pelvis — Numbness, weakness, or difficulty walking.

How is the diagnosis made?

The diagnosis is made after a tissue sample is examined under the microscope by a pathologist. The first sample is usually obtained through a biopsy — a small piece of the tumor removed with a needle or through a small incision. After a biopsy confirms the diagnosis, patients are typically treated first with chemotherapy and sometimes radiation therapy. The tumor is then surgically removed and sent to pathology as a resection specimen for complete evaluation.

Under the microscope, embryonal rhabdomyosarcoma is made up of immature muscle cells called rhabdomyoblasts. These cells have small, round, dark-staining nuclei — a feature pathologists describe as hyperchromatic, meaning the nucleus absorbs more stain and appears darker than normal. The nucleus is often pushed to the side of the cell (described as eccentric), and a small amount of pink cytoplasm — the material filling the cell’s body — is visible beside it. More mature rhabdomyoblasts may have abundant eosinophilic (pink) cytoplasm and a distinctive shape, sometimes described as “strap cells” or “tadpole cells”. The tumor typically shows a mix of primitive-looking cells in a loose background and areas of denser cellularity. Mitotic figures — cells caught in the act of dividing — are usually present and reflect the tumor’s active growth.

To confirm the diagnosis, the pathologist uses immunohistochemistry (IHC) — a laboratory test that uses antibodies to detect specific proteins inside tumor cells. Embryonal rhabdomyosarcoma characteristically shows positivity for muscle markers, including desmin, myogenin, and MyoD1. These proteins confirm that the tumor cells have a skeletal muscle identity. Myogenin staining in embryonal rhabdomyosarcoma is typically patchy (present in only a subset of cells), which helps distinguish it from alveolar rhabdomyosarcoma, where myogenin staining is characteristically strong and diffuse throughout the tumor.

Molecular testing is performed to identify specific genetic changes associated with rhabdomyosarcoma subtypes. This is done using fluorescence in situ hybridization (FISH) or next-generation sequencing (NGS). In embryonal rhabdomyosarcoma, the key finding is the absence of a FOXO1 gene rearrangement — the translocation that defines alveolar rhabdomyosarcoma. Confirming the absence of this fusion is important because it distinguishes embryonal from alveolar rhabdomyosarcoma, which has a worse prognosis and requires different risk-adapted treatment. Your pathology report will state which molecular test was performed and its result. Once the diagnosis is confirmed, imaging — typically MRI of the primary site and CT of the chest, abdomen, and pelvis — is performed to assess the full extent of disease.

Histologic grade

Pathologists do not assign an FNCLCC grade to embryonal rhabdomyosarcoma. The FNCLCC system — the standard grading system for most soft tissue sarcomas — is not applied here because embryonal rhabdomyosarcoma is already defined as a high-grade cancer, regardless of the individual scoring components used in that system. Your pathology report will therefore not include a numeric grade, and this is expected and appropriate for this diagnosis.

Tumor size

Tumor size is measured at its greatest dimension in centimeters. Tumors smaller than 5 cm are less likely to have spread to other parts of the body and are associated with a better prognosis. Tumor size is also used to determine the pathologic tumor stage (pT). The size is recorded from the surgically removed specimen, not from a biopsy.

Tumor extension

Embryonal rhabdomyosarcoma typically starts within a soft tissue site but can grow into surrounding structures as it enlarges. This spread beyond the original location is called tumor extension. The pathologist carefully examines all tissue submitted with the resection specimen to determine whether tumor cells have grown into adjacent muscles, bones, nerves, blood vessels, or organs. Extension into surrounding structures increases the pathologic tumor stage (pT) and may require more extensive surgery, additional radiation therapy, or both to achieve local control.

Treatment effect

Because most patients with embryonal rhabdomyosarcoma receive chemotherapy and/or radiation therapy before surgery, the pathologist evaluates the surgical specimen to determine how well the tumor responded to pre-operative treatment. This assessment is called the treatment effect.

The pathologist estimates the percentage of the tumor that is non-viable (dead) versus viable (still alive). A tumor that is 90% or more non-viable indicates an excellent response to pre-operative therapy and is associated with a better outlook. When a significant proportion of viable tumor remains, the treatment team may discuss additional treatment after surgery. Your report will describe the estimated percentage of viable and non-viable tumor.

Lymphovascular invasion

Lymphovascular invasion means that tumor cells are present inside blood vessels or lymphatic channels within or around the tumor. Blood vessels carry blood throughout the body, while lymphatic channels carry lymph fluid toward nearby lymph nodes. When tumor cells enter either type of vessel, they have a route to lymph nodes or to distant organs, such as the lungs. Lymphovascular invasion is therefore associated with an increased risk of spread and is an adverse feature when present. Your pathology report will state whether lymphovascular invasion was identified.

Perineural invasion

Perineural invasion means that tumor cells are growing along or around a nerve. Nerves run throughout the soft tissues, and tumor cells that reach them can use the nerve pathway to extend into surrounding tissue beyond the main tumor mass. This increases the risk that the tumor will grow back in the same area after treatment. Your pathology report will state whether perineural invasion was identified.

Surgical margins

A margin is the edge of the tissue removed during surgery. Margins are evaluated only after a surgery that removes the entire tumor — not after a biopsy, which takes only a small sample. The pathologist examines all cut surfaces of the specimen to determine whether tumor cells are present at the edges.

• Negative margin — No tumor cells are present at the cut edge. This suggests the tumor was completely removed in that area. The distance from the nearest tumor cells to the margin may also be recorded.
• Positive margin — Tumor cells are present at the cut edge, raising concern that some cancer remains in the body. A positive margin increases the risk of local recurrence and may lead to a recommendation for additional surgery or radiation therapy.

Lymph nodes

Lymph nodes are small immune organs found throughout the body. Cancer cells can spread from the tumor to nearby lymph nodes through lymphatic channels — a process called metastasis. If lymph nodes were removed during surgery, the pathologist examines them under the microscope and records whether tumor cells are present.

Your report will state the total number of lymph nodes examined and whether any contain tumor cells. It may also describe the size of tumor deposits within nodes and whether cancer cells have broken through the outer wall of a lymph node into the surrounding tissue — a finding called extranodal extension, which is associated with a higher risk of recurrence. Lymph node involvement determines the pathologic nodal stage (pN1) and influences decisions about the intensity of additional treatment.

Biomarker and molecular testing

For embryonal rhabdomyosarcoma, there are currently no established biomarkers that directly guide selection of a specific targeted drug, in contrast to cancers such as breast cancer (HER2, ER/PR) or colorectal cancer (KRAS, MMR). The molecular testing performed on this tumor — FISH or NGS to assess for a FOXO1 rearrangement — is primarily a diagnostic test used to confirm the subtype (described above under “How is the diagnosis made?”) rather than a treatment-directing biomarker.

Research into molecular targets in rhabdomyosarcoma is ongoing. In patients with relapsed or refractory disease, comprehensive molecular profiling using next-generation sequencing (NGS) may be performed to identify genetic changes that could make a patient eligible for a clinical trial or targeted therapy. Your oncologist will discuss whether molecular profiling is appropriate in your situation. For more information about biomarker testing in cancer, visit our Biomarkers and Molecular Testing section.

Pathologic stage (pTNM)

The pathologic stage describes how far the cancer has spread, based on examination of the surgical specimen. It uses the internationally recognized TNM staging system, which considers the primary tumor (T), lymph node involvement (N), and distant metastasis (M). Metastasis to distant organs is typically determined by imaging rather than by pathology. In general, higher numbers indicate more advanced disease.

Children with rhabdomyosarcoma are also assigned a clinical grouping using the Intergroup Rhabdomyosarcoma Study Group (IRSG) system, which considers tumor location, the extent of surgical removal, and whether lymph nodes or distant sites are involved. This grouping guides the intensity of chemotherapy and radiation. Your oncologist will explain which system applies to your situation.

Tumor stage (pT)

The pT stage depends on tumor size and the site in the body where the tumor started.

Head and neck:

• pT1 — Tumor is 2 cm or smaller.
• pT2 — Tumor is more than 2 cm but 4 cm or smaller.
• pT3 — Tumor is more than 4 cm.
• pT4 — Tumor has grown into surrounding structures such as bones of the face or skull, the eye, major blood vessels of the neck, or the brain.

Trunk and extremities (chest, back, abdomen, arms, legs):

• pT1 — Tumor is 5 cm or smaller.
• pT2 — Tumor is more than 5 cm but 10 cm or smaller.
• pT3 — Tumor is more than 10 cm but 15 cm or smaller.
• pT4 — Tumor is more than 15 cm.

Thoracic visceral organs (organs within the chest or abdomen):

• pT1 — Tumor is confined to one organ.
• pT2 — Tumor has grown into the connective tissue surrounding that organ.
• pT3 — Tumor has grown into at least one adjacent organ.
• pT4 — Multiple tumors are present.

Retroperitoneum (deep abdominal cavity behind the organs):

• pT1 — Tumor is 5 cm or smaller.
• pT2 — Tumor is more than 5 cm but 10 cm or smaller.
• pT3 — Tumor is more than 10 cm but 15 cm or smaller.
• pT4 — Tumor is more than 15 cm.

Orbit (tissue around the eye):

• pT1 — Tumor is 2 cm or smaller.
• pT2 — Tumor is more than 2 cm but has not grown into the bones surrounding the eye.
• pT3 — Tumor has grown into the bones surrounding the eye or other bones of the skull.
• pT4 — Tumor has grown into the eye itself or surrounding structures such as the eyelids, sinuses, or brain.

If no viable tumor is found in the resection specimen after pre-operative treatment, the stage is recorded as pT0. If the specimen cannot be reliably assessed, it may be listed as pTX.

Nodal stage (pN)

• pN0 — No cancer found in any lymph nodes examined.
• pN1 — Cancer found in one or more regional lymph nodes.
• pNX — Lymph nodes were not assessed.

What is the prognosis for embryonal rhabdomyosarcoma?

Embryonal rhabdomyosarcoma has a better overall prognosis than alveolar rhabdomyosarcoma. With modern multimodal treatment, five-year survival rates for localized disease are generally 70–90%, and outcomes for low-risk disease (small, completely resected tumors at favorable sites) are even higher. Metastatic disease carries a substantially worse prognosis, with five-year survival rates below 30%.

The most important prognostic factors include:

• Stage and tumor site — Tumors at favorable sites — such as the orbit, non-parameningeal head and neck, and genitourinary tract (excluding the bladder and prostate) — have the best outcomes. Tumors at unfavorable sites — such as parameningeal, extremity, bladder, prostate, and trunk locations — carry a higher risk of recurrence.
• Completeness of surgical resection — Complete removal with negative margins is the most important favorable factor for local control. Residual disease after surgery (positive margins) significantly worsens prognosis.
• Age — Children between the ages of 1 and 9 generally have better outcomes than infants under 1 year or older adolescents and adults.
• Tumor size — Tumors under 5 cm have a better prognosis than larger tumors.
• Lymph node involvement — Spread to regional lymph nodes increases the stage and worsens the prognosis.
• Response to treatment — A good pathologic response (90% or more tumor necrosis) after pre-operative chemotherapy is a favorable sign.
• Molecular subtype — The absence of a FOXO1 fusion (confirming the embryonal subtype) is itself a favorable prognostic feature compared to FOXO1-fusion-positive alveolar rhabdomyosarcoma.

What happens after the diagnosis?

Treatment for embryonal rhabdomyosarcoma requires a multidisciplinary team including a pediatric or adult oncologist, radiation oncologist, and surgeon experienced in sarcoma. The treatment plan depends on age, tumor site, stage, and risk group.

For nearly all patients, treatment begins with multi-agent chemotherapy. The standard backbone for low- and intermediate-risk disease is vincristine, actinomycin D, and cyclophosphamide (VAC), often with modifications based on risk group. Chemotherapy reduces the tumor before local treatment and addresses microscopic spread elsewhere in the body.

Surgery aims to remove the tumor with negative margins whenever this is feasible without unacceptable functional loss. At favorable sites such as the orbit, complete resection is often possible and curative when combined with chemotherapy. At other sites, limb-sparing or organ-sparing surgery is prioritized.

Radiation therapy is added when complete surgical removal is not possible, when margins are positive, or at specific high-risk sites such as parameningeal tumors. In the orbit, radiation often replaces surgery as the primary local treatment to preserve vision and the eye.

After completing treatment, regular follow-up imaging is required at defined intervals to monitor for recurrence. Surveillance typically continues for at least five years. Late recurrences are less common in embryonal rhabdomyosarcoma than in alveolar rhabdomyosarcoma, but vigilance is maintained throughout the follow-up period. Patients should also be monitored for long-term effects of chemotherapy and radiation, particularly those treated during early childhood.

Questions to ask your doctor

Your pathology report contains important information that will guide your care. The following questions may help you prepare for your next appointment.

• Was this confirmed as embryonal rhabdomyosarcoma, and was FOXO1 testing performed to rule out the alveolar subtype?
• What is the pathologic stage, and has the cancer spread to lymph nodes or other organs?
• How large is the tumor, and where exactly did it start?
• Did the tumor extend into any surrounding structures such as bone, nerves, or blood vessels?
• Were the surgical margins negative? If positive, what treatment is recommended?
• Were lymph nodes removed and examined, and were any positive for cancer?
• Was lymphovascular or perineural invasion identified in the specimen?
• If pre-operative treatment was given, what percentage of the tumor was dead, and what does the treatment response mean for further management?
• What risk group does this tumor fall into, and how does that affect the treatment plan?
• What chemotherapy regimen is planned, and for how long?
• Is radiation therapy part of the treatment plan?
• What follow-up schedule is recommended, and which parts of the body will be imaged?
• Are there clinical trials available for this diagnosis?